It is necessary to have special functions for a window for an optical use, to be fitted to a vacuum apparatus used, for example, in the case of carrying out an optical measurement in a range of vacuum ultraviolet to infrared or in a further wide wavelength range in ultra-high vacuum. In particular, two of these functions are required, i.e. that a window material having a high percent transmission as to a wide range wavelength is used and that a fitted part of the window material is in such a connection state that the fitted part is resistant to baking at a temperature as high as possible. For such a use, CaF.sub.2, LiF, BaF.sub.2, NaCl, etc. have been used as a window material because of their higher transmittance over a wide range. Since a flange for fitting to a vacuum apparatus and a material for the window material have different coefficients of thermal expansion, however, the window material is subject to strain and broken when the baking temperature is higher.
Thus, in order to absorb the difference in coefficients of thermal expansion between the flange and window material, the following method is employed. That is, this method comprises bonding the flange and window material through a frame and adhesive material. In this method, firstly, the frame to be fitted with the window material is bonded to the flange. Then, an Au film is formed on the window material. This is formed by uniformly coating liquid gold onto an edge (surface in contact with frame) of the window material in such a manner that the coating is not uneven and then subjecting to a heat treatment (500 to 600.degree. C.). When an adhesive material is mounted on a butt surface of the frame and window material and subjected to a heat treatment, the frame and the window material are bonded through the adhesive material. The thus resulting window material for an optical use is resistant to ultra-high vacuum by sealing with the above described adhesive material. This method has an advantage that stress is absorbed by a frame when using a material having a relatively close coefficient of thermal expansion to that of the optical window material as an adhesive material and a relatively deformable metal for the frame.
A window for an optical use, capable of being resistant to high vacuum and being subjected to baking at a high temperature by the use of such a frame and adhesive material, can be obtained, but has the following problem.
Namely, as an intensity of a light to be transmitted through a window is increasing with development of a light source, the window material as described above cannot neglect heat generation due to energy loss during transmission of the light. In order to decrease the energy loss, the thickness of a window material can be decreased, but a thickness of at least a certain value is required for resisting vacuum and accordingly, decrease of the thickness of the window material is naturally limited. Furthermore, when the thickness of a frame is increased, strain is directly transferred to the window material and the absorbing effect of strain of the adhesive material is thus decreased. Accordingly, it is necessary to decrease the thickness of the frame. However, heat generated in the window material is transmitted to a vacuum flange through the frame, cooling being effected therefrom, and if the thickness of the frame is decreased, accordingly, the cooling efficiency is markedly lowered.
The present invention has been accomplished in view of the problems of the prior art and aims at providing a window for an optical use, having excellent transmission property over a range of from vacuum ultraviolet to infrared or further wide wavelength range as well as excellent durability and capable of transmitting a light having a large energy and being fitted to an ultra-high vacuum apparatus, and a process for the production of the same.